Solar Powered Lantern

This is a quick DIY electronics project I set for myself so I could brush up on my circuit design and some building techniques. I bought a couple copper lanterns off ebay and fitted them with a circuit similar to the outdoor solar lights you can buy for your yard. Much of the circuit design is based on one of the simple designs on Evil Mad Scientist. That’s a great page to get started with.

I modified the circuit so that the lanterns could operate indoors near a window. Most solar lights, including the one on Evil Mad Scientist, are designed to be kept outdoors. So the solar panel gets a lot more light than one that’s kept indoors. If you use a 3V solar panel to match up with the 2.4V battery, the panel needs to get direct sunlight to charge the battery, and the light will turn on well before it gets dark. I decided to use a 6V solar panel. This way it charges the battery with only indirect light through a window. One potential issue is that a 6V solar panel under direct sunlight could overcharge the 2.4V batteries. Though, I’ve been running this circuit for over a year without any problems.

Assembly is pretty straight forward. You can see how I built it in the montage video at the bottom of this post. If you want the light to run outside, you’ll need to take extra precautions for waterproofing.

I built a solar powered lamp out of an old kerosene lamp, I use two aa rechargeable batts and a yellow flicker led. My problem is the light works fine all day long, when ever I put something over the solar cell the light works fine, however at night the light does not come on. When I shine a light on the panel the light comes on and stays on all night. I have tried a lot of fixes but nothing works, any idea’s. Thanks Bill

In this design, the led should turn on when no light is on the solar panel. It sounds like your lantern is doing the opposite, so the led turns on when the solar panel is lit. Is that the behavior you’re seeing? If so it’s possible that you’re using a NPN instead of a PNP transistor. I’ve also found that the type of solar panel makes a difference. For example, the thin flexible solar panels do not work with this design because they do not allow any current to pass through them then they’re not lit.

I finally got the circuit working, however I still have a problem. When I touch the led leads to the double a battery leads it is nice and bright, but when the circuit turns on the light it is very dim, could it be the resistor is too big? I get the same voltage at the led leads that I get at the batteries but a dim led output. Any ides’s. Thanks Bill

I had exactly this problem when I was using flexible (amorphous) solar panels instead of crystalline panels. Are you using amorphous panels? They have great performance, but don’t let any current flow through them when they’re not lit. The PNP transistor needs a small amount of current going to its base for it to open. This current is provided by the battery, through the solar panel, when the voltage on the panel is low. Amorphous cells don’t appear to allow this current flow. At least that was my diagnosis at the time. I’m sure an electrical engineer could give a more accurate analysis.

Bill

Dec 1, 2011, 17:43

Alex, I am using a solar panel from a yard light, it puts out about 5 volts. Thanks again for your help. Bill